Short answer: localing a value implicitly undefines it. Thus, you can think of this as doing:

local $class->db_Main->{ AutoCommit };
$class->db_Main->{ AutoCommit } = undef;
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And, since autocommit is used as a boolean value, undef is treated in the same manner as zero, so it's like setting it to zero.

Personally, I like to be explicit about such things, so I would say

local $class->db_Main->{ AutoCommit } = 0
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But that's just me. It's not necessary for any purpose other than to avoid having someone ask this question. In my opinion, though, that's a worthwhile reason to do it.